Cultivated sunflower (Helianthus annuus L.) is being grown as an increasingly important oilseed crop in many temperate, semi-dry regions of the world.
The cultivated sunflower is a major worldwide source of vegetable oil. Oil types of sunflowers contain 40 to 48 percent or more oil in the seed. Sunflower oil is valued as an edible oil because of its high unsaturated fat level and light color. Sunflower oil is used for salads, cooking oil or margarine. The protein content of sunflower meal prepared from seeds after oil extraction is useful as livestock feed. The seeds from both oil and confectionery varieties of cultivated sunflower are useful as bird food.
The parasitic plant broomrape (Orobanche Spp.) has become a limiting factor for sunflower crops in infested countries. The decrease in crop yield can reach 95% in an affected field. Particular Orobanche species afflicting sunflower include Orobanche aegyptiaca Pers., O. ramosa L., O. minor Sm., O. cumana Wallr. and O. cernua Loefl.
Orobanche cumana Wallr. (also known as O. cernua Loefl.) is a severe pest in sunflower in eastern Europe and has been spreading through southern Europe. Over the past few years the progression of this parasitic plant, its introduction into new countries and the development of new and more virulent races have all been observed. Orobanche presents a worldwide risk and some species such as O. minor have appeared as exotics in the United States.
Starting in 1996, the emergence of three new races of broomrape was observed in fields in Turkey, Spain and Bulgaria. By 2000, the number of affected fields in these areas had sharply increased. Some producers had stopped growing sunflower because of the significant reduction in grain yield experienced.
These weeds are obligate root holoparasites. Orobanche species are very difficult to eliminate because except for their flower parts, they live in the soil, their seeds are minute and prolifically produced, easily dispersed and very long-lived. Thus, herbicides presently used in sunflower generally provide inadequate control.
Means of controlling Orobanche include biological control agents, isolation of the genes responsible for Orobanche resistance in sunflower and development of resistant sunflower lines.
It has been reported that Fusarium oxysporum f. sp. orthoceras may be a potential agent for the biological control of Orobanche cumana (Thomas-Heiko, et al., (September 1998) Biological Control. 13(1):41-48). However, application methods and uniform doses under field conditions have yet to be determined.
The Or3 gene confers resistance to Orobanche attack but is known to be efficacious only against race C. At least six race variants of Orobanche have been reported (Pérez-Vich, et al., (2004) Theor. Appl. Gen. 109:92-102; Antonova, et al., (1996) Weed Research 36(2):113-121).
Identification and utilization of additional sources of resistance to Orobanche is beneficial in managing race shifts and minimizing production loss.